French mathematician and philosopher René Descartes (1596–1650) was living in Holland in 1637 when he published his first major work, Discourse on Method. In this treatise, he extends mathematical methods to science and philosophy, asserting that all knowledge is the product of clear reasoning based on self-evident premises. This idea, that there are certitudes, provided the foundation for modern philosophy, which dates from the 1600s to the present.

Descartes may be best known for the familiar phrase “I think, therefore I am” (Cogito ergo sum, in Latin). This assertion is based on his theory that only one thing cannot be doubted, and that is doubt itself. The next logical conclusion is that the doubter (thinker) must, therefore, exist. The correlation to the dictum (I think, therefore I am) is dualism, the doctrine that reality consists of mind and matter: Since the thinker thinks and is, he or she is both mind (idealism) and body (matter, or material). Descartes concluded that mind and body are independent of each other, and he formulated theories about how they work together. Modern philosophers have often concerned themselves with the question of dualism.

Descartes’s other major works include Meditations on First Philosophy (1641), which is his most famous, and Principles of Philosophy (1644). His philosophy became known as Cartesianism (from Cartesius, the Latin form of his name).